Behavioral development of the Norway rat is analyzed on several levels of psychobiological organization. Relations between growth, body temperature regulation, and behavior will be studied with allometric analyses of individual pups and huddles. Regulatory behaviors should emerge differentially under different environmental conditions. Similarly, other developmental landmarks, such as onsets of filial attraction and nest egression can be related to parameters of growth and behavioral expression. Ontogenetic studies of behavioral thermoregulation and oxygen consumption will be conducted. The infants' high frequency vocalizations will also be considered as a form of behavioral thermoregulation, including an oxygenation hypothesis which models the pups' cries to mechanisms common to Respiratory Distress Syndrome in human infants. Behavioral preferences for metabolically-equivalent situations will tested, providing an empirical foundation for relating and discriminating hedonics and homeostasis. Early learning will be studied with a recentlydiscovered operant conditioning method with thermal reward for infants. Developmental differences in hedonic constraints appear to affect learning. We will extend our analyses of parent-offspring relations through studies of weaning in rats, which we view as an empirical testbed for studying the achievement of independence. We will expand our understanding of offspring regulation of maternal responsiveness by studying the diminution of maternal salt appetite in relation to lactation, maternal licking, and other aspects of caregiveing. New studies of weaning of the parent are proposed.